1. Field of the Invention
This invention relates to the non-invasive sensing of blood glucose levels and, more particularly, to a non-invasive probe suitable for use with a non-invasive blood glucose monitor for patients with diabetes.
2. Description of the Prior Art
It is generally known in the art that radiation, particularly near-infrared radiation over a range of wavelengths, can be projected in a non-invasive manner on a portion of the body of a patient. The resulting radiation emitted from that portion of the body, either scattered or transmitted after absorption and scattering, can be detected and processed to derive an expression of the detected radiation as a function of wavelength and, therefrom, the concentration of blood glucose. Since the detected radiation is a continuous signal covering all of the wavelengths in the range of interest, it is necessary for further analysis to separate the intensities of radiation at the various individual wavelengths, or smaller bands of wavelengths, to extract the desired blood glucose level information.
U.S. Pat. Nos. 5,070,874 and 5,460,177 describe methods for the non-invasive measurement of blood glucose levels. In general, these methods use a spectrophotometer to measure the absorbance of the near-infrared radiation at different wavelengths across the range of interest. The absorbance plotted against the wavelengths constitutes a spectrum. By analyzing the spectrum, the blood glucose levels, or changes thereto, can be determined. As the blood glucose levels vary, the detected spectrum changes slightly.
In order to make the measurements discussed above, the radiation must be transmitted from a radiation source to the skin of a patient and the detected radiation received back from the patient must be collected and carried to the spectrophotometer for further analysis. Prior art fiber optic bundles are typically arranged in a completely coherent or ordered manner, in order to provide an image transfer capability, or are arranged in a completely incoherent or unordered manner and function as a simple light conduit for applications in which an imaging capability is unimportant. The much more expensive coherent fiber bundles are commonly used in medical and industrial probes, such as endoscopes and borescopes. Much less expensive incoherent fiber bundles are suitable for use in connection with non-imaging detectors and for specialized illuminators, such as those used in microscopy. Prior art fiber optic bundles that combine transmitting and receiving fibers are typically used some distance from their intended target and rely on reflection of the transmitted light by the target surface to illuminate the receiving fibers.
The prior art does not in any way address the high level of mechanical and thermal isolation of the return or receiving fibers necessary for the proper function of the fiber optic probe in glucose detection. Microbending stresses in the receiving fibers induced by various vibration sources from both within and outside the glucose monitor can produce intensity variations in the spectral signal being received from the patient and can thereby induce errors in the resulting spectral data. Thermal changes along the length of the receiving fibers, globally, localized or transient, have experimentally been shown to produce similar variations in the spectral signal, resulting in errors in the spectral data received from the patient.